Classic modalities for treatment of diseases such as human cancers, autoimmune diseases, viral, microbial, parasitic and fungal diseases include surgery, radiation, chemotherapy, antibiotics or combination therapies. However, extended therapy with these agents may cause greater morbidity than the underlying disease. Alternate therapies for preventing or treating human diseases are greatly needed. In the past decade immunotherapy has emerged as a new and promising method for treating cancers.
Immunotoxins are being used in the treatment of diseases such as cancers, autoimmune diseases, viral, microbial, parasitic and fungal diseases. In one approach, antibodies targeting tumor associated antigens or tumor specific antigens are administered as a form of treatment. To augment the therapeutic efficacy of the antibodies, the antibodies are often conjugated to cytotoxic drugs, toxins or radioisotopes. Monoclonal antibodies coupled to protein toxins, called immunotoxins, are being examined in numerous clinical trials for treatment of cancer and autoimmune diseases (Rybak, S. M., et al. (1991) Immunology and Allergy Clinics of North America 11:359-380). Subsequent to cell surface binding by the monoclonal antibody, the toxic protein subunit crosses the membrane surrounding the cytosol to reach the intracellular substrate. Ricin, for example, enzymatically inactivates ribosomes, inhibiting protein synthesis and causing cell death (Endo, Y., et al (1987) J. Biol. Chem. 262:5908-5912; Olsnes, S., et al. (1973) Biochemistry. 12:3121-3126; Olsnes, S., et al. (1976) J. Biol. Chem. 257:3985-3992). How the hydrophilic enzyme crosses into the cytosol is unknown, although endocytosis and intracellular routing to the proper compartment are required (Johnson, V. G., et al. (1991)"Intracellular Routing And Membrane Translation Of Diphtheria Toxin And Ricin" In Intracellular Trafficking of Proteins. 183-225 C. J. Steer and J. A. Hanover (eds) Karger (Basel)). The Golgi apparatus appears to be one compartment through which ricin must pass en route to the cytosol. Native ricin efficiently routes through the Golgi apparatus to the cytosol due to galactose binding sites on the ricin B chain (Gonatas, N., et al. (1975) Exp. Cell Res. 94:426-431; Hudson, T. H., et al. (1991) J. Biol. Chem. 266:18586-18592; Sandvig, K., et al. (1991) J. Cell Biol. 115:971-981; Youle, R. J., et al. (1987) J. Biol. Chem. 262:4676-4682). When the ricin B chain is removed and enzymatically active A chain is linked to monoclonal antibodies reactive with cell surface molecules such as the transferrin receptor, much less efficient entry into the cytosol ensues (Youle, R. J., et al. (1982) J. Biol. Chem. 257:1598-1601). Although the immunotoxin is rapidly endocytosed via the transferrin receptor, it does not traffic such that the enzymatically active A chain rapidly reaches the cytosol. In addition to ricin B chain, some drugs that cause alterations in the Golgi apparatus such the ionophore, monensin, and lysosomotropic amines cause a large increase in cell sensitivity to the immunotoxins (Casellas, P., et al. (1984) J. Biol. Chem. 259:9359-9364). Chloroquine (Laurent, G., et al. (1986) Blood. 67:1680-1687), a lysosomotropic agent, and the ricin B chain (Grossbard, M. L., et al. (1992) Blood 79:576-585) have been tested in man for their ability to improve the anti-cancer activity of immunotoxins.
Retinoids are a large family of molecules encompassing over three thousand members. Retinoic acid, a member of the retinoid family, is a morphogen that defines certain cell fates during development and has the potential to treat cancer by inducing tumor cell differentiation (Petkovich, M. (1992) Ann. Rev. Nutr. 12:443-471; Thaller, C., et al. (1991) In Retinoids: 10 Years On 89-108 J. H. Saurat (ed) Karger/Basel). Retinoid acid binds the retinoic acid receptor (RAR) causing it to form heterodimers with the retinoid X receptor (RXR) and induce gene transcription (Chambon, P., et al. (1991)"The family of retinoid acid nuclear receptors" In Retinoids: 10 Years On. 10-27 J. H. Saurat (ed), Karger/Basel; Kliewer, S. A., et al. (1992) Nature. 355:446-449; Zhang, X. K., et al. (1992) Nature. 355:441-446). In addition to the well accepted role of retinoids in transcription activation, some retinoids may have direct effects on cell second messengers (Evain-Brion, D., et al. (1991) "Retinoid Acid & Cellular Signal Transduction" In Retinoids: 10 Years On. 46-55; J. H. Saurat(ed) Karger/Basel).
Additional therapies that potentiate the toxicity of the immunotoxins to a high degree of specificity for the affected target cell would greatly facilitate treatment of human diseases with these immunotherapeutic agents.